1. Field of the Invention
This invention relates to a light signal reading method used when information is optically reproduced from a recording medium on which the information is magnetically recorded, and an apparatus therefor.
2. Description of the Prior Art
In recent years, in the field of high density recording of information, the opto-magnetic recording system using the magneto-optical effect and capable of rewriting has been regarded as being promising instead of the fixed information type recording system using a so-called optical disc or the like.
Heretofore, a reading optical system as shown in FIG. 1A of the accompanying drawings has generally been used for the reproduction of the opto-magnetic record as described above. A parallel light beam 1 emitted from a light source (not shown) such as a semiconductor laser is made into a light beam linearly polarized in a certain direction by a polarizer 2, passes through a beam splitter 3 and an objective lens 4 and enters a magnetic film 5 while assuming the form of a spot. The magnetic film 5 is formed on a substrate 9 of glass or resin by a technique such as sputtering, and information is recorded thereon as a variation in the direction of magnetization of the magnetic film. The light beam 1 which has entered the magnetic film 5 is reflected with its plane of polarization rotated in opposite directions by the magneto-optical kerr effect correspondingly to the direction of magnetization of the magnetic film 5. For example, assuming that the plane of polarization of the light beam reflected by a downwardly facing magnetized area is subjected to a rotation of .theta.K, the plane of polarization of the light beam reflected by an upwardly facing magnetized area is subjected to a rotation of -.theta.K.
Now, where the above-mentioned incident light beam 1 is S-polarized light as shown in FIG. 1B of the accompanying drawings, if the direction of transmission of polarized light through an analyser 6 is disposed in a direction (Q direction) perpendicular to the above-mentioned direction of polarization -.theta.K, the reflected light from the upwardly facing magnetized area is intercepted by the analyser 6 and only the reflected light from the downwardly facing magnetized area is detected by a photodetector 8 with only the amplitude component in the direction of transmission of polarized light transmitted through the analyser 6 being passed through a condensing lens 7, whereby the information magnetically recorded on the magnetic film 5 can be digitally read.
However, the kerr rotation angle .theta.K by the magneto-optical Kerr effect is a minute angle of the order of 0.1.degree. and moreover, the power of the reproducing light must be made low so as not to erase the magnetically recorded information on the magnetic film, and the transmitted signal component from the analyser 6 is very minute. Therefore, for example, where an ordinary PIN photodiode or the like is employed as the photodetector 8, heat noise becomes dominant and deterioration of the signal to noise ratio (S/N ratio) for reproduction has become a problem. Also, where an avalanche photodiode (hereinafter referred to as the APD) having an internal increasing action is used for photodetection, an increase in signal power can be achieved by increasing the increase rate until the shot noise becomes equal to or greater than the heat noise, but sufficient improvement of the S/N ratio could not be achieved due to the increase noise peculiar to the APD. Further, means for eliminating the instability of the increase rate of the APD is necessary, and this has led to a disadvantage that the construction becomes complicated.